1. Field of the Invention
The present invention relates to a resist composition used in a manufacturing process of semiconductor devices, such as IC, manufacture of circuit substrates for liquid crystals, thermal heads and the like, and lithographic process of other photo-fabrication, and also the invention relates to a pattern-forming process using the resist composition. In particular, the invention relates to a resist composition suitable for exposure with an immersion projection exposure apparatus using far ultraviolet rays of 300 nm or less as the light source, and a pattern-forming process using the resist composition.
2. Description of the Related Art
With the progress of fining of semiconductor elements, shortening of the wavelengths of exposure light source and increasing of the numerical aperture of the projection lens have advanced, and now exposure apparatus of NA 0.84 using an ArF excimer laser having the wavelength of 193 nm as the light source have been developed. As generally known, these can be expressed by the following equations:(Resolution)=k1·(λ/NA)(Depth of focus)=±k2·λ/NA2 wherein λ is the wavelength of an exposure light source, NA is the numerical aperture of the projection lens, k1 and k2 are the coefficients concerning the process.
For further higher resolution by the shortening of the wavelengths, an exposure apparatus with an F2 excimer laser having the wavelength of 157 nm as the light source has been studied, but the materials of lens for use in the exposure apparatus for shortening of the wavelengths and the materials of resist are extremely restricted, so that the realization of the reasonable manufacturing costs of the apparatus and materials and quality stabilization are very difficult, as a result, there are possibilities of missing an exposure apparatus and a resist having sufficient performances and stabilities within a required period of time.
As a technique for increasing resolution in optical microscope, a so-called immersion method of filling between an objection lens and a sample with a liquid of high refractive index (hereinafter also referred to as “immersion liquid”) has been conventionally known.
As “the effect of immersion”, the above resolution and depth of focus can be expressed by the following equations in the case of immersion, taking λ0 as the wavelength of the exposure light in the air, n as the refractive index of immersion liquid to the air, and NA0=sin θ with θ as convergence half angle of the ray of light:(Resolution)=k1·(λ0/n)NA)0 (Depth of focus)=±k2·(λ0/n)NA02 
That is, the effect of immersion is equivalent to the case of using exposure wavelength of the wavelength of 1/n. In other words, in the case of the projection optical system of the same of NA, the depth of focus can be made n magnifications by immersion. This is effective for every pattern form, and can be combined with super resolution techniques such as a phase shift method and a deformation lighting method.
As the apparatus applying this effect to the transfer of micro-fine image pattern of semiconductor element, JP-A-57-153433 and JP-A-7-220990 are known, but resists suitable for immersion exposure techniques are not disclosed in these patents.
It is appointed in JP-A-10-303114 that the control of the refractive index of immersion liquid is important as the variation of the refractive index of immersion liquid causes the deterioration of a projected image due to the wave surface aberration of exposure apparatus, and controlling the temperature coefficient of the refractive index of an immersion liquid to a certain range, and water added with additives for reducing surface tension or increasing the degree of surface activity are disclosed as a preferred immersion liquid. However, the specific additives are not disclosed and resists suitable for the technique of immersion exposure are not also discussed.
The latest technical progress of immersion exposure is reported in SPIE Proc., 4688, 11 (2002), and J. Vac. Sci. Techol. B, 17 (1999). When an ArF excimer laser is used as the light source, it is thought that pure water (refractive index at 193 nm: 1.44) is most promising in the light of the safety in handling, the transmittance and the refractive index at 193 nm. When an F2 excimer laser is used as the light source, a solution containing fluorine is discussed from the balance of the transmittance and the refractive index at 157 nm, but a sufficiently satisfactory solution from the viewpoint of environmental safety has not been found yet. From the extent of the effect of immersion and the degree of completion of resist, it is thought that immersion exposure technique will be carried on an ArF exposure apparatus earliest.
From the advent of resist for KrF excimer laser (248 nm) on, an image-forming method called chemical amplification is used as the image-forming method of resist for compensating for the reduction of sensitivity by light absorption. To explain the image-forming method of positive chemical amplification by example, this is an image-forming method of exposing a resist to decompose an acid generator in the exposed area to thereby generate an acid, utilizing the generated acid as the reactive catalyst to change an alkali-insoluble group to an alkali-soluble group by the bake after exposure (PEB: Post Exposure Bake), and removing the exposed area by alkali development.
In immersion exposure, a resist film is exposed through a photomask, with an immersion liquid between the resist film and the optical lens, to transfer the pattern of the photomask to the resist film, but it is anticipated that the chemical reaction (acid catalyst type desorbing protection reaction, development reaction) caused inside the resist during or after exposure might be influenced by the immersion of the immersion liquid to the inner part of the resist film. However, the extent of the influence and the mechanism are not known yet.
When chemical amplification resist is applied to immersion exposure technique, the acid on the surface of the resist generated at exposure time migrates to the immersion liquid, and the acid concentration of the surface of the exposed area changes. This is supposed very similar to acid deactivation of the exposed area caused by basic contamination of a trace amount of several ppb level from the environment at the time of post exposure time delay (PED:Post Exposure time Delay) between exposure and PEB, which became an issue at the beginning of the development of a chemical amplification positive resist, but the influence of immersion exposure on a resist and its mechanism are still unclear. When a chemical amplification resist that causes no lithographic problem in general dry exposure is subjected to immersion exposure, the deterioration in sensitivity is seen as compared with the time of general dry exposure and the improvement is required. Further, if a large amount of acid is eluted from the resist surface to the immersion liquid at the time of immersion exposure, the pollution of the object lens of an exposure apparatus is anticipated, therefore it is required to reduce the elution of generated acid to the least.